Genentech’s cancer drug Avastin, when given along with chemotherapy, raises the risk of blood clots, a company-sponsored study confirmed. Data from five clinical trials showed that 3.8 percent of patients with advanced cancers given Avastin with chemotherapy developed blood clots in their arteries, compared with 1.7 percent of those given chemotherapy alone. The study was published Tuesday in the Journal of the National Cancer Institute. Avastin is the South San Francisco company’s biggest product, generating $1.75 billion in sales last year.